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• A gas turbine is basically a constant
flow engine. The fuel is burned and
serves to add heat to compressed air
which then expands through the blades
of a turbine.
• Use the same impulse and reaction
principle as in steam turbine
• Use in aircraft locomotive and stationary power generation
• Only 1/3 the power produced is useful for work, the rest used
to keep itself (driving the compressor) running.
• Gas turbines have been used in small sizes to produce power
for portable and standby generators, fire pumps, and
• Large units are used for electricity generation in stationary
The Single open cycle Turbine Unit
Without Heat Exchanger
Principle of operations Open cycle
• The simple gas turbine operates by drawing air into
a compressor, compressing it and then discharging
the air into a combustion chamber.
• Fuel is added and burned, heating the air further.
• This hot air is then expanded through a turbine,
providing available power. Air is then exhausted to
atmosphere (open cycle).
• Not self starting; must use starting motor to rotate to
20 to 30% before fuel is turned on
• The air used to drive the turbine is in a
close cycle. It is separated from the
• Exhaust from turbine goes to the intake
of the compressor. It absorbs heat from
the furnace and expanded in the
• The turbine and compressor are on the
same shaft so that the compressor obtains
its drive from the turbine.
• About two-thirds of this power is used
to drive the compressor, but the
remainder is available to drive a load or
• The efficiency of such a machine depends
primarily upon the temperature to which the
air can be raised before entering the turbine.
• The output power, or power rating, of the
machine, depends upon the mass of hot gas
flowing through the gas turbine in a given
time per period.
• A gas turbine output increases when using
air of maximum density.
• "simple" - it has no heat exchanger or regenerators.
• "open-cycle" - the air used to drive the turbine is
drawn from the atmosphere and returned to the
atmosphere after use.
• "closed-cycle" would use the same air again and
• "single shaft" - is self-explanatory, there is only one
shaft. The gas turbine and the compressor are
coupled together mechanically on this shaft.
Gas Turbine Characteristics
• Gas turbines have certain
characteristics which distinguish them
from other types of prime movers,
some of these have proved to be
advantages and some disadvantages.
High Power to Weight Ratio
• Compared to a steam plant, a gas turbine is
much smaller and lighter for the same power.
This is a considerable advantage since the
building housing a gas turbine can be much
smaller and needs only light foundations.
Construction cost is lower.
Low Installed Cost
• The capital cost of a gas turbine is
lower per watt of output than a steam
Low Maintenance and Operating
• The absence of reciprocating motion
minimizes wear of moving parts.
Lubricating oil consumption is low.
Because of gas turbine simplicity,
many auxiliaries required in a steam
plant are eliminated.
Minimum Cooling Water
• Gas turbines require a minimum of
cooling water and can be operated
economically with a closed circulating
water system using air cooled heat
exchangers. This makes gas turbines
ideal for use wherever water is scarce
Rapid Start-Up and Loading
• A gas turbine can be started almost
instantaneously (Power up in 20 minutes). It
does not require slow and elaborate
preparations as does a steam turbine supplied
by a steam generator.
• The gas turbine is ideal for standby duties, such
as for fire pumps and emergency electricity
generators where instant starting is essential, or
where remote operation by telemetry is required.
• Low Thermal Efficiency
• This is the major drawback of the gas
turbine. The thermal efficiency of the simple,
open-cycle gas turbine is considerably below
that of a steam plant, a diesel engine or a
• Auxiliary equipment must be fitted, such as
regenerators, in order to make the gas
High Noise Level
• Unless special precautions are taken to
baffle the exhaust, the gas turbine is a
• NOx – run at high temperature thus
• (NOx is produced around 1500 -1600°C)
Typical temperatures which exist in
a gas turbine.
• Simple, Open-Cycle, Single-Shaft Gas Turbine
Objective #3 Regeneration
• The purpose of the regenerator on the gas turbine is to
improve the cycle efficiency by recovering some of the
heat which would otherwise pass to waste with the
• The regenerator is placed in the air flow after the
compressor and before the combustion chamber.
• The compressor works most effectively with cold air. The
heat recovered from the exhaust gas reduces the fuel-quantity
required to produce the same load.
Combined Steam - Gas turbine Plants
• Gas turbines can be used in
conjunction with steam boilers in
• Waste heat boiler
• Combined cycle
• pressurize the boiler furnace with the air leaving the
compressor and then pass the hot gases from the
boiler through a gas turbine to drive the compressor
and produce mechanical drive for electricity
• In this type of arrangement the gas turbine
contributes to the total plant capacity and the overall
plant economy is improved.
• The high furnace pressure at which a boiler of this
type operates permits a reduction in boiler size.
Gas Turbine Combined with a Steam Plant
• The high energy content of the gas
turbine's exhaust may be used in several
ways to increase the overall efficiency of a
• The most common example is a waste
heat boiler where the turbine exhaust
gases are used to heat water to supply
steam for some other purpose.
• Another method uses the exhaust gases,
which are at a temperature of about 480ºC
and contain considerable excess oxygen,
as preheated air for a boiler furnace.
• Fuel may be used in conjunction with the
hot air to further burn the remaining
excess oxygen to produce steam.
• By employing a combined cycle, the thermal
efficiency can be raised to 60% and the design
retains several advantages over a straight steam
• A combined cycle plant can be started up and
shut down rapidly.
• Several gas turbines can be used with one
steam turbine to produce more power, and
higher efficiencies can be maintained across a
wider range of loads (for example, 25% to 100%
Combined cycle advantages
• Faster start-up: full load can be obtained
in 30 minutes.
• Lower cooling water requirements, by as
much as 67%.
• Lower atmospheric pollution from oxides
of nitrogen (NOX).
• Lower cost of investment per kilowatt
Objective 4 Gas turbine operation
• Turbine capacity is depended on the
density of air.
• Any change in atmospheric conditions that
affect the temperature and pressure of air
will affect the performance of the gas
• The mass of inlet air flowing through the
compressor and, after combustion,
through the turbine, is highly dependent
on altitude and air temperature.
• Plant capacity will be higher in cold
weather as the density of the air will
increase, allowing more air into the
turbine, increasing mass flow.
• Changes in atmospheric pressure will
similarly affect turbine output.
• A turbine operating at sea level will have
higher density air (more mass) at its inlet
than one operating in higher altitude.
Gas turbine starting
• Manual – small plant
• Semi-automatic – operator starts
auxiliary and automatic control starts
• Automatic – program starting
• engine starts to rotate the system;
when the speed is ~30% of max rpm,
the combustor will ignite
• when the rpm is 50% of max, started
engine shut off
• if the system does not reach operating
speed in a certain time, abort start up.
• Governor senses load-shaft speed to
adjust fuel to combustor to control the
• Small unit uses fly ball governor; big
units employ more sophisticated
• Over speed trip – same as in steam
• Gas turbine is not self starting. It
requires electric or engine driven
starter. Speed has to be 30% of
maximum before it can be self-support.
• Lubrication – could use pump driven
by the gas turbine shaft. Auxiliary oil
supply is needed for cooling the
bearings in case of failure.
• Blade cleaning system is used to get rid of
foreign material deposited on blades.
Medium used could be detergent.
• In extreme case, crushed walnut shells
mixed with air could be used!
• Pre-charged halon or CO2 cylinders are
located in various parts of the system and
can discharge automatically.
• halon breaks down into corrosive and toxic
byproducts when it is exposed to surface
temperatures above 482°C (900°F).
• Water mist could also be used to for
• Loss of 0.5% per 1000 hours of service
or 4.4% per year is expected.
• The exhaust gases of all engines contain some
constituents that are harmful in the atmosphere. If these
constituents are discharged in large enough quantities
for a long enough period of time then they will violate Air
• Generally, gas turbines produce trace amounts of carbon
monoxide and sulphur dioxide, but they can be
significant generators of oxides of nitrogen (NOX) and
• The operator should maintain proper combustion in the
turbine and use any equipment required to meet Air
Environmental Considerations 2
• Noise pollution by the gas turbine can be a
problem, but it may be overcome or
reduced by acoustic insulation and
• Operators should pay special attention to
hearing protection regulations when
entering buildings housing gas turbines in